Sludge, waste solids and scum collector mechanisms are commonly used in wastewater treatment tanks to remove debris and settled sludge from the bottom of collection tanks, to skim the floating grease and solids from the surface of collection tanks, and for removing grit and trash from wastewater and process streams. Typically, such collector and conveyor mechanisms include a pair of spaced apart continuously articulating chains trained over collector chain sprockets which are mounted on cross shafts supported on the sidewalls of the tank or equipment, and elongated transverse collector flights, scrapers and skimmers supported between the chains. One of the sprockets is driven so that the chain loops revolve to provide a succession of collector flights along a bottom of the mechanism as well as along the surface of the water in the mechanism basin to accomplish the collection, conveying, and skimming functions. Successive collector flights push and discharge scum, sludge and debris into collection areas of the tank or mechanism for removal.
Series 700 chains have been one of the process equipment mainstays in the municipal and industrial water and wastewater treatment industry since the early 1900s. The Series 700 chain is used extensively in process waste solids and bio-solids removal equipment including rectangular chain and scraper sedimentation clarifiers, storm water collectors and clarifiers for grit, sand and debris removal in chain and flight grit collectors, chain and bucket grit collectors, grit elevators, and grit clarifiers, and for screening and removal of suspended debris, trash and rags in bar screens and trash rakes.
Until the early 1970s, Series 700 chain was only available in cast or fabricated steel. While this chain has a high initial ultimate strength and application working load (42,000 pounds and 4,200 pounds respectively), the chain suffers from rapid wear and loss of strength primarily from oxidation of the link material, chemical and biological induced corrosion, abrasive wear from contact with iron oxide fines (from corrosion within the chain link components), and from contact with grit and silica fines in the process streams. The average surface life for most equipment applications is seven years or less.
In 1975, Rexnord Corporation of Milwaukee, Wis. introduced the first plastic series 700 Series chain molded from acetal plastic to eliminate the rapid wear caused by corrosion and oxidation. While the plastic chain provides excellent service in most lightly loaded chain and scraper collection equipment and clarifiers, the chain has significant limitations with regard to ultimate strength and working load (6,000 pounds and 2,500 pounds respectively). This often limits effective service for CS (chain and scraper) collector equipment applications 250 feet or less in length.
All plastic chain and plastic chain pin materials have little abrasion resistance which renders standard plastic chains vulnerable to severe abrasive attack, and makes them generally unsuitable for sustained grit collector service. In some cases, the lack of abrasion resistance can also make plastic chain unsuitable for sustained cross collector service in rectangular collectors. In addition, the higher load requirements for grit collectors and screening equipment are often well above the rated working load of this chain, and may even exceed the ultimate strength of the plastic chain causing frequent service outages and catastrophic equipment failures.
Plastic materials are susceptible to attack from ultraviolet radiation and sunlight, can become brittle in severe cold, and have a high coefficient of expansion and contraction due to seasonal air temperature variations, and variable ambient water temperature. This makes the standard plastic chain generally unsuitable for applications where they are frequently exposed to the atmosphere, in other non-submerged applications such as storm water collectors, or in other applications with high temperature, or very high (caustic) or very low (acid) chemical pH concentrations.
As an alternate to plastic chain, and for corrosives applications with high working loads and temperature, some manufacturers developed welded or fabricated Series 700 chain link design, sometimes manufactured from grade 300 or 400 stainless steel bars, rounds and/or plate, in an attempt to reduce the effects of corrosion, and to a lesser extent, the effects of abrasion.
While fabricated stainless steel chains can approach the ultimate strength and working load of conventional cast chains, such designs, depending on the grade of stainless steel used, they have significant strength and working load limitations. Chain manufactured from 400 stainless steel, while more corrosion resistant than conventional cast steel chain, has a much lower corrosion resistance and strength than a cast 316 stainless steel link, even when the 400 stainless steel link is heat treated. Unlike 316 stainless steel alloys, grade 400 stainless steels undergo corrosive attack when exposed to a variety of acids, alkalis, chlorides and sulfide compounds, and gases commonly found in water and wastewater treatment.
All welded chain experiences carburization of the link material at the weldments, which results in stress risers and embrittlement of the link material. These factors contribute to stress fractures of the link material, and limit the ultimate strength and working load capacity of the these chains.
Welded and fabricated chains are labor intensive to manufacture which often increases the cost to the consumer beyond the point that it is cost effective when compared to conventional cast chain. To compensate for the additional cost of manufacturing, some fabricated chain designs utilize smaller chain pin diameters, thinner length sidebars, and less expensive materials. These result in reducing the ultimate strength, working load and service life of the chain.
From the beginning of chain link design, conventional steel chain designs have had non-rotating chain pins that are mechanically locked into the chain sidebars by means of a T-head shaped chain pin, or by using a riveted “HEAD” chain pin that is tightly press fit in place. These conventional chain pins often have flat or tapered ends machined into the opposite end of the chain pin which further locks the chain pin in place with the opposite chain sidebar. The chain pin cannot be installed from the opposite direction. On all other conventional chain designs, both the riveted head and T-head end of the chain pin and the opposite (cottered) end of the chain pin have an interference fit, and must be pressed into the chain sidebars by mechanical means. This requires the use of substantial mechanical force for inserting the pin, and some mechanical method for preventing the sidebars from bending and spreading as the chain pin is forced into the sidebars. This often requires the use of two people to complete this assembly task. Locking of the chain pin causes the load and wear to always be concentrated only on one side of the chain pin outside diameter and only on one side of the chain pin inside diameter as the chains articulate around sprockets. Since the chain pin is fixed in place, none of the rotating wear is distributed to the chain pin bosses at the open end of the link. The concentration of wear and load to reduced surface areas causes accelerated and one-sided premature failure of the chain pin outer diameter and the chain link barrel inner diameter.
In order to eliminate the corrosion and wear problems, and all of the other most common problems associated with conventional sand-cast, fabricated and plastic Series 700 chain in the water and wastewater industry, it is desirable to provide a unique investment cast stainless steel 720S chain link. The new chain meets and exceeds all current standards for Series 700 class welded steel chain, cast chain and attachments as established for the water and wastewater industry per American National Standard ASME B29.21M-1996.
Although investment casting has been alluded to in previous patents for other cast chain designs, this manufacturing process, especially as it relates to the use of stainless steel alloys, has never been perfected and used previously. Casting stainless steel is significantly different from casting conventional steel alloys.
The investment casting process does not readily release, disperse or diffuse the heat of the molten metal through the mold media, unlike the coarse grain sand used in the conventional sand casting process. Prior to the development of the present invention, no one has been able to control the variables of the investment casting process to achieve both high uniform strength and close dimensional tolerances required for the offset sidebar Series 700 chain link. Significant heat concentrations from the casting process must be dispersed from the chain link sidebars, and the round pin bosses at both the pin boss (open end) and the chain barrel (closed end).
It is further desirable to provide an investment casting process for stainless steel alloys that eliminates the heat concentrations and the formation of stress risers within and about the chain sidebars, the chain boss (open end of the link) and the chain barrel (closed end of the link), and that eliminates shrinkage and dimensional variations within and between the holes for the chain pin. It is also desirable to provide an investment casting process that holds the dimensional tolerances between the open end and the closed end pin holes within each link to within +/−0.010 inches, exceeding the +/−0.018 inches dimensional American engineering standard allowed for conventional sand cast steel chains.
In the formation of the chain link, molten stainless steel must be delivered into the mold in a rapid and uniform manner to assure uniform isotropic grain structure throughout the casting. The unidirectional flow pattern of the mold design also assures a linear (not transverse) grain structure parallel to the depth of the chain sidebar. In addition, the link must be cooled slowly and uniformly to prevent carburization and the formation of stress risers within the link, and to prevent the link from warping during the cooling stage. The link must also be cooled slowly and uniformly to prevent shrinkage of the link material that would compromise the close tolerances required between the link components.
It is additionally desirable to provide an investment casting mold structure which will overcome the aforementioned problems encountered in casting single or multiple chain links.